1. Field of the Invention
The present invention relates to an image forming apparatus which has a function of measuring color of a measurement image.
2. Description of the Related Art
An image forming apparatus has image qualities such as granularity, in-plane uniformity, character quality, and color reproducibility (including color stability). With the proliferation of multi-color image forming apparatuses these days, the color reproducibility is sometimes said to be the most important image quality.
People remember empirically expected colors (of human skin, blue sky, and metal in particular), and will feel uncomfortable if acceptable ranges thereof are exceeded. Such colors are called memory colors. Ensuring the reproducibility of memory colors has been often required when outputting photographs.
Aside from photographic images, office users may feel unconformable about a color difference between document images displayed on a monitor and output documents. Graphic arts users are also pursuing the color reproducibility of computer graphics (CG) images. The demand for the color reproducibility (including stability) of an image forming apparatus is therefore increasing.
To satisfy the users' demand for the color reproducibility, Japanese Patent Application Laid-Open No. 2004-086013 discusses an image forming apparatus which reads a measurement image (patch image) formed on a sheet by using a measurement unit (color sensor) arranged on a sheet conveyance path. This image forming apparatus can give feedback to process conditions, including an amount of exposure and a developing bias, based on the reading result of the patch image by the color sensor, whereby a certain density, gradation, and color can be reproduced.
However, the color sensor discussed in Japanese Patent Application Laid-Open No. 2004-086013 causes lower accuracy in color measurement due to factors such as variations in the output of a light source when the ambient temperature changes. Thus, a white reference plate may be arranged in a position opposed to the color sensor. The white reference plate can be measured by the color sensor to perform processing for correcting a measurement value of the color sensor.
However, during measurement of the patch image, if the white reference plate is arranged in the position opposed to the color sensor, the white reference plate is irradiated with light transmitted through the sheet on which the patch image is formed, and thereby discolored. The reason is that the white reference plate contains a substance that discolors due to a photo-oxidation effect.
Thus, there has been a problem that as the number of times of irradiation of the white reference plate with light increases, the discoloration of the white reference plate caused by the irradiating light gradually advances, leading a gradual increase in errors in measurement values.
Another problem is that when a sheet passes through a conveyance path between the color sensor and the white reference plate, foreign substances such as paper dust can adhere to the surface of the white reference plate, causing an error in the measurement value of the white reference plate by the color sensor.